This is a proposal to study collaboratively the effects of hydroxyurea on hemoglobin F production and the clinical state in sickle cell disease. Patients with severe clinical manifestations will, after informed consent, be given small amounts of the drug for pharmacokinetic studies. They will then be given a dose of 15 mg/Kg/day with escalation of the dose until minimal hematologic toxicity is observed. The dose will be lowered and will be maintained for 20-24 months to determine the effect of the drug on the production of hemoglobin F, on hematologic parameters, and on the clinical status of the patient. Observations for toxicity and carcinogenesis will be made. The data will be analyzed to determine: 1. differences in dose of the drug on the hematological parameters in patients with sickle cell disease. 2. the effect of differences in the phenotypic expression of sickle cell disease on the response to hydroxyurea 3. Correlation of the response to hydroxyurea as measured by hematologic variables and the clinical response as measured by changes in clinical state.